Determining and/or monitoring the presence of certain chemical species within an environment, e.g., pollutants, toxic substances, and other undesirable compounds, is becoming of increasing importance with respect to such fields as health, environmental protection and resource conservation.
There exists very sophisticated and complicated systems which are capable of detecting the presence of, for example, a substance in the atmosphere, even down to as low a level as a trillionth of a gram. However, many devices are impractical for field applications. For example, in analyzing water or soil samples for the presence of harmful substances, the samples are generally collected from the field and then taken to the lab and subjected to analysis using, for example, a gas chromatograph and/or a mass spectrometer. These types of analysis equipment, while very sophisticated and precise, are not practical for use in the field, require a substantial capital investment, and often take a long period of time for completion of analysis, i.e., often up to several days.
There are devices which are less expensive and smaller in size than those discussed above which provide for a detection of a change in mass. These devices are known as piezoelectric sensors, such as surface acoustic wave (SAW) or quartz crystal microbalance (QCM) devices. They are based on a piezoelectric crystal. By employing an alternating voltage to an interdigital transducer on the piezoelectric crystal, there results a surface acoustic wave. The propagation velocity of this surface acoustic wave is a sensitive probe of near surface mass changes and elastic moduli. Thus, when a substance adsorbs onto the surface of the SAW devices, there is produced a response. SAW devices are capable of detecting mass changes as low as about 100 pg/cm.sup.2. Similarly, an alternating voltage at the two opposite electrodes on a QCM (AT-cut quartz) induces a thickness shear mode whose resonance frequence is proportional to mass. However, while these devices are very sensitive mass detectors, they are not inherently selective with respect to different substances.